Fix remaining Qt6 build warnings (narrowing, nodiscard, qHash)

Clears the 9 non-deprecation warnings from the Qt6 build:
- qHash(QColor): hash rgba() (unambiguous QRgb) instead of name(), and
  use the size_t seed signature on Qt6 (guarded for Qt5). Fixes the
  ambiguous-overload warning in terminalstripmodel.h.
- Two qsizetype->int narrowings in brace-init: explicit static_cast<int>
  (elementscene.cpp, terminalstrip.cpp).
- main.cpp: keep the QtConcurrent::run QFuture in a [[maybe_unused]]
  variable (nodiscard).
- qetapp.cpp: guard the stylesheet load on QFile::open() succeeding
  (nodiscard) instead of ignoring the result.

sources/qetapp.cpp

@@ -1697,10 +1697,11 @@ void QETApp::useSystemPalette(bool use) {
 				);
 	} else {
 		QFile file(configDir() + "/style.css");
-		file.open(QFile::ReadOnly);
-		QString styleSheet = QLatin1String(file.readAll());
-		qApp->setStyleSheet(styleSheet);
-		file.close();
+		if (file.open(QFile::ReadOnly)) {
+			QString styleSheet = QLatin1String(file.readAll());
+			qApp->setStyleSheet(styleSheet);
+			file.close();
+		}
 	}
 }